Neurons of the retina are organized with impressive precision in a pattern of several distinct laminae. This exact arrangement is absolutely essential for normal eye function. To identify and characterize the genetic pathways responsible for the generation of this neuronal pattern, we are using a recently developed vertebrate model system, the zebrafish. Over the last several years we isolated nearly 50 mutations which affect development of the zebrafish eye. A group of these mutations, defining three independent loci, cause drastic disarrangement of the retinal neurons. In the mutant retinae, all normal types of neurons are present but occupy grossly abnormal positions. In this proposal, we focus on two neuronal patterning loci, ome and nok. Our studies have shown that ome mutations affect previously unknown cell-cell interactions in the retinal neuroepithelium. A defect in these interactions is accompanied by a loss of polarity in the neuroepithelial cells. Our current goal is to determine which cell type produces the cellular signal involved in these interactions. It will be accomplished by constructing genetically mosaic animals. Further more, we would like to investigate whether the same cell-cell interactions are also affected in nok. The nok and ome phenotypes are similar, suggesting that this may be the case. The ome and nok mutations provide an excellent opportunity to gain insight into the molecular mechanisms involved in formation of neuronal patterns. We plan to use the positional cloning strategy to characterize the ome and nok loci at the molecular level. Cloning of these genes will allow us to characterize the cellular and subcellular distribution of their transcripts and protein products and make predictions about their biochemical function. Sequence information will also make it possible to relate them to components of other cell-cell signaling cascades. Remarkable progress in the field of vertebrate genomics, combined with the advantageous characteristics of zebrafish embryology, carry a promise that studies of zebrafish mutants will reveal key developmental mechanisms potentially related to human eye disorders.